Over half a century ago, Caprican scientists created the "CYbernetic LabOr uNit". Given the nickname "Cylons", these self-aware bipedal robots were manufactured to make life easier within the Twelve Colonies. Eventually the Cylons revolted and decided to kill their human masters. After the long and bloody struggle that was the Cylon War, an armistice was declared. The Cylons withdrew from Colonial space to a planet of their own. For some 40 years, nothing was heard of them, and no apparent attempt was made by the Colonials to keep an eye on them. The only contact expected between the two was created by the Colonial government through planned annual diplomatic meetings at Armistice Station. However, while the Colonials sent a repesentative to the station every year, the Cylons did not.
During their forty years of isolation, the Cylons developed a bioengineered human form that is indistinguishable from humans, down to human personalities (affection, jealousy, sadness, anger, sense of humor, etc.).
These Humanoid Cylons are primarily human in biology. The exception to this is in their entire nervous system. Simply put, Humanoid Cylons don't have any organic nerve cells at all. Instead, in the place of nerve cells, the Humanoid Cylons have billions and billions of microscopic "silica pathway" nanomachines linked together in a cohesion subroutine, which includes the Humanoid Cylon brain. All of these nanomachines are configured to mimic neurons in both function and aesthetics. They are sheathed in a bioengineered organic polymer that gives them the appearance of genuine nerve tissue, including brain matter. So essentially Humanoid Cylons are nanotechnology-based lifeforms encased in genetically engineered human bodies.
Normal human bodies can't function with a mechanical nervous system, so the Cylons perform very subtle, but specific changes to the genetic code of the humanoid models during engineering that very slightly alters the metabolic processes of the human body to a degree that is not even noticeable, but enables the body to accept the nanomachine nervous system.
The bodies of Humanoid Cylons sometimes appear to have tremendous stamina and strength, and appear to be designed to destroy or resist commonly dangerous human diseases, although they are not immune from all contagions. In keeping with their desire to be a better human, however, the Cylons did not or could not further improve on other characteristic design flaws of the human body. However, they are heavily fortified to resist damage from intense radiation fields that would kill a human after short exposure.
Humanoid Cylons can be programmed to believe that they are truly human. These are known as 'sleeper agents'. Sleeper agents have a low-level Cylon personality that can conduct operations while placing the human sleeper personality "on-hold." Most agent copies, particularly Numbers Five and Six, imitate human behavior, but are fully aware of their true nature and behave more or less like a human "spy."
Humanoid Cylons are also susceptible to the same emotional traumas and clevages that their human counterparts are, further proving that personalities are a realistic part of their psyche, despite the presence of the Cylon portion of their personality. A case in point is a copy of Number Six, known as Gina, who was repeatedly physically and sexually assaulted by the crew of Pegasus. These repeated assaults lead to Gina's near-catatonic state at the time Peagsus discovered the Fleet.
The psychology of Humanoid Cylons is based on what they simply refer to as "projection." Essentially an enhanced form of a self-induced and controlled hallucination, it's how they choose to see the world around them. Cylons choose the see their environment in any form they wish, whenever they wish. For example, if a Cylon were standing in a hallway, they could see it as a forest filled with birds, trees, and sunlight. The aesthetics is what gives them pleasure.
The majority of the Cylons follow a monotheistic religion (with the exception of the Cavil model, which appears to be an atheist).
Although they cannot do so remotely or wirelessly, Humanoid Cylons can interface with a computer system or computer network through subdermal physical contact with input cables connected to the devices. Caprica-Valerii connected to Galactica's computers by this method when she inserted a fiber-optic cable through her left palm and into her arm. This subdermal fiber-optical link is an interface with the nanotech nervous system. The process involves cutting an opening into the Cylon's palm. They can perform this through subdermal physical contact if required or, preferably, through a Cylon data port. It must be noted that this is not a common method of data access.
A Humanoid Cylon's nanomachine brain is certainly human in appearance, but is still susceptible to damage from certain types of radiation. Their program can be corrupted, thereby sending the wrong signals throughout the body's systems; causing fever and eventually death.
To operate, the nanomachines are dependent on the biochemistry of the human shells in a symbiotic connection. This means that if the body dies, the nanomachines become inert. However, the artificial intelligence software is capable of transmitting itself into another nanotech organism encased in a human body (of the appropriate model) i.e. upon the death of a Humanoid Cylon's body, it transfers its consciousness into another copy of itself. This "resurrection" is limited by distance, signal integrity, and proximity to a "resurrection node" or, in space, a "Resurrection Ship."
All Humanoid Cylons share a collective knowledgebase of data from other active or deactivated Humanoid Cylons. However, it is not of a collective consciousness "hive mind" nature.
At least one model of Cylon female has shown the capacity to bear children seeded by human males. The Cylons tested numerous methods of sexual reproduction, yet these failed due a well known flaw in the Humanoid Cylon design: a genetic quirk caused by the genetic recoding makes procreation between Cylons impossible. So in an attempt to subvert this deficiency, they try to interbreed with humans, creating hybrid offspring. "Farms" were set up across all the occupied Colonies where survivors, specifically, young fertile women of child-bearing age, were rounded up, placed under heavy sedation and turned into "baby machines" through artificial insemination (There are presumably also farms for male humans, plus in vitro experiment labs, etc.). However, this method has been unsuccessful thus far. Another drastically different approach is taken by attempting to conceive a child through a bond of love. It seems that Karl Agathon and Caprica-Sharon Valerii's love-child is the lone success story of the relatively short history of Cylon sexual reproduction.
There are twelve models of Humanoid Cylons, each with many copies, probably tens or hundreds of thousands. The twelve models are based on archetypes determined by the Cylons that form what they perceived as the specific kinds of human behavior and/or personality, distilled into twelve varieties. Each Humanoid Cylon has the same face, stature, hair, eye and skin color, gender, and other biometrics as all the others of his or her model. This point technically makes each copy a twin of itself, but there are some models that choose to make cosmetic differences. As well, the copies start out with the same basic personality, but grow more distinct due to their individual experiences.